Ischemic disease remains a major cause of morbidity and mortality in the USA and worldwide, and is particularly problematic in diabetics. Cell therapies have been demonstrated to enhance local vascularization and perfusion in a variety of models, but patient improvement in the clinical trials to date has been modest. This likely relates to the small percentage of transplanted cells that engraft successfully and participate in rebuilding the vasculature, and the current limited knowledge of the mechanism(s) by which the cells exert their effects. The specific hypothesis to be addressed in this project is that the ability of transplanted endothelial progenitor cells to build a vascular network and relieve tissue ischemia can be dramatically enhanced by providing a sustained release of appropriately primed cells into the hypoxic tissue from a biomaterial vehicle. This hypothesis will be evaluated with the following set of aims: (1) Determine if appropriate design of a vehicle microenvironment can regulate endothelial progenitor cell gene expression, migration through the material and dispersion into the surrounding tissue, (2) Quantify the ability of a sustained delivery and tissue repopulation by endothelial progenitor cells to relieve acute and chronic peripheral ischemia in SCID mice, and determine the mechanism(s) of this effect, and (3) Examine the ability of material-mediated endothelial progenitor cell delivery to enhance recovery from peripheral ischemia in the context of diabetes, using a STZ induced mouse and an alloxan-induced rabbit model of diabetes. The data and systems arising from these studies may impact several areas of biology and engineering research, and lead to clinical strategies to revascularize ischemic tissue. Perhaps most importantly, a new approach will be developed in this project that aims to effectively repopulate ischemic tissues with cells competent to orchestrate neovascularization. This approach will be investigated in the context of the peripheral ischemia, but this approach would also find utility in the treatment of coronary artery disease and other situations involving tissue ischemia. We anticipate that autologous cells would initially be used in these applications, and the clinical experience developed to date in endothelial progenitor isolation and expansion will directly apply to the cell source aspects of this approach. These studies will also improve the current understanding of the role of the two cell populations - EPCs and OECs in vascularization, and this may lead to completely new strategies of neovascularization in the future.